This invention is directed to the fields of molecular biology and medical diagnostics and therapeutics.
Prostate cancer is the most prevalent form of human cancer and the third most common cause of cancer death in men. Methods for early detection and treatment of this disease would decrease the rate of prostate cancer deaths. Tumor-associated proteins, which are proteins expressed by malignant cells but few others, are useful as targets for detection and for intervention. Several proteins associated with prostate cancer have been identified, including prostate specific antigen (PSA).
Immunotherapy is a potent new weapon against cancer. Immunotherapy involves evoking an immune response against cancer cells based on their production of target antigens. While humoral immune responses against cancer cell antigens have uses, it is preferred to invoke a cell-mediated immune response against cancer cells. Immunotherapy based on cell-mediated immune responses involves generating a cell-mediated response to cells that produce particular antigenic determinants. Cancer cells produce various proteins that can become the target of immunotherapy. Certain cancers produce novel proteins, for example as a result of mutation, that are immunogenic. However, investigators also have discovered tumor infiltrating lymphocytes that specifically recognize un-mutated proteins of cancer cells. For example, Rosenberg et al. have shown that tumor infiltrating lymphocytes target and recognize antigenic determinants of the protein MART-1, produced by both normal melanocytes and malignant melanoma cells. Furthermore, active or passive immunotherapy directed against MART-1 or peptides of it that bind to MHC Class I molecules (epitopes of HLA A2, in particular) results in the destruction of melanoma cells as well normal cells that produce MART-1. Y. Kawakami et al., J. Immunol. 21:237 (1998).
Novel cancer antigens are expected to provoke an immune response because the immune system will recognize them as non-self proteins. However, the ability of the immune system to invoke an immune response against an un-mutated self protein was surprising because the immune system develops tolerance to self proteins. It is believed that this immune response is directed against antigenic determinants that normally are not exposed to the immune system in sufficient quantity to invoke either tolerance or an immune response. In cancer, however, these determinants no longer escape detection by the immune system. This may result from increased presentation of the determinants by MHC Class I molecules.
The cell-mediated immune response involves the activity of Major Histocompatibility Complex molecules. In humans, this complex is called the “HLA” (“Human Leukocyte Antigen”) complex. In mice, it is referred to as the “H-2” complex. The major histocompatibility complex includes three classes of proteins, MHC class I, MHC class II and MHC class III. MHC class I molecules are expressed on the surface of nearly all nucleated cells. They present antigen peptides to TC cells (CD8+). There are three MHC class I gene loci in humans, HLA A, HLA B and HLA C. Each locus is highly polymorphic. Therefore, a person may have up to six different kinds of HLA molecules on the surface of their cells. MHC Class II proteins are expressed primarily on antigen presenting cells such as macrophages, dendritic cells and B cells, where they present processed antigenic peptides to TH cells. There are three MHC Class II gene loci in humans, HLA DP, HLA DQ and HLA DR. MHC class m proteins are associated with various immune processes, and include soluble serum proteins, components of the complement system and tumor necrosis factors. J. Kuby, Chapter 9, IMMUNOLOGY, Third Edition W.H. Freeman and Company, New York (1997).
In cancer cells as well as healthy cells, MHC class I molecules present epitopes from endogenous proteins for presentation to TC cells. HLA A, HLA B and HLA C molecules bind peptides of about 8 to 10 amino acids in length that have particular anchoring residues. The anchoring residues recognized by an HLA class I molecule depend upon the particular allelic form of the HLA molecule. A CD8+ T cell bears T cell receptors that recognize a specific epitope when presented by a particular HLA molecule on a cell. When a TC cell that has been stimulated by an antigen presenting cell to become a cytotoxic T lymphocyte contacts a cell that bears such an HLA-peptide complex, the CTL forms a conjugate with the cell and destroys it.
The presentation of peptides by MHC Class I molecules involves the cleavage of an endogenously produced protein into peptides by the proteasome, its processing through the ER and Golgi apparatus, its binding to the cleft in an MHC Class I molecule through the anchor residues of the peptide and ultimate presentation on the cell surface. Depending upon the particular anchor residues, among other things, certain peptides may bind more tightly to a particular HLA molecules than others. Peptides that bind well are referred to as “dominant” epitopes, while those that bind less well are termed “subdominant” or “cryptic” epitopes. Dominant epitopes of either self proteins or foreign proteins evoke strong tolerance or immune responses. Subdominant or cryptic epitopes generate weak responses or no responses at all. It is hypothesized that tighter binding by dominant epitopes to HLA molecules results in their denser presentation on the cell surface, greater opportunity to react with immune cells and greater likelihood of eliciting an immune response or tolerance.
Investigation has shown that in the case of the MART-1 protein, a self protein, the immune system generates the greatest CTL response against subdominant or cryptic epitopes. Y. Kawakami et al. 1997 Immunol. Res. 16:313. It may be that in cancer cells subdominant or cryptic epitopes are presented much more densely or in greater amounts than is normal; consequently, the immune system encounters these previously undetected epitopes, recognizes them as foreign and generates an immune response against them. Whatever the reason, exposing the immune system to large amounts of subdominant or cryptic epitopes of self proteins as a means of eliciting an immune response against cells that produce that protein is a key element of cancer immunotherapy. Of course, eliciting an immune response against a self protein will result in the destruction of both cancerous cells and healthy cells. Therefore, for such immunotherapy to succeed, the healthy cells must either be non-essential for life or have functions that are replaceable by other therapies. In the case of prostate cancers and breast cancer, surgical removal of the prostate or breast, respectively, is a frequent therapeutic intervention. In such cases, most or all of the tissue displaying a prostate or breast antigen will be a tumor cell.